KENNEDY SPACE CENTER, FLA. - The aft segment of a Solid Rocket Booster is transported to the Vehicle Assembly Building (in the background). The segment will be mated with other segments for use on the launch of Space Shuttle Atlantis on the second Return to Flight mission, STS-121. SRBs are used as matched pairs and each is made up of four solid rocket motor segments. Two SRBs provide the main thrust to lift the Space Shuttle off the pad to an altitude of about 150,000 feet. In addition, the two SRBs carry the entire weight of the External Tank and orbiter and transmit the weight load through their structure to the Mobile Launcher Platform. Each booster has a thrust (sea level) of approximately 3,300,000 pounds at launch, and provide 71.4 percent of the thrust at liftoff and during first-stage ascent.